Use defaults to reference current keymint version

The KeyMint HAL will soon be updated for a new version. To make this
process easier, add a cc_defaults and rust_defaults that references
the "current" version, and use this elsewhere. This should hopefully
mean that a future version bump only needs to happen in the defaults.

Test: TreeHugger
Change-Id: If7dd0c5778acb92177e16fd4fb4a04dcb837ad06